High-elasticity press felt

ABSTRACT

A high-elasticity dewatering felt for use in e.g. papermaking and cellulose machines wherein the elastic properties of the felt are obtained by admixture thereinto of high-molecular thermoplastic elastomer materials, alternatively high-molecular cross-linked urethane-based elastomer materials, which elastomer materials are capable of stretching to at least twice their original length and subsequently, after relief of the load thereon, rapidly return to substantially their original length.

BACKGROUND OF THE INVENTION

In machines for the production of paper, paper pulp and similarproducts, the fibrous web is dewatered partly by being squeezed betweentwo press rollers. For economical reasons it is generally desirable toremove the majority of the moisture of the fibrous web in the presssection. In the last decades, intensive research has led to entirely newtypes of felts and cloths designed for dewatering purposes in the presssection of paper machines. Owing to these new products, it has becomepossible to meet increasing demands on dewatering efficiency. At thesame time, the machine speeds have become higher, with the result thatthe demands on the runability and wear resistance of these products alsohave increased.

The older type of press felts which comprised a woven, conventionalproduct which was felted and napped has practically disappeared from themarket and is replaced by needled felts. As a rule, these needled feltshave a basic structure in the form of a base weave. This base weave mayalso be replaced by yarn material in one direction only of the two feltdirections. The base structure of the needled felt may also be totallyvoid of yarn materials. Up-to-date needled press felts usually consistof one compressible part which is positioned closest to the fibrous webthat is to be dewatered, and one less compressible part which isdesigned to receive and carry away the water that is squeezed out of thefibrous web. When a felt of this kind is used in a papermaking machine,the same area of the felt passes the press nip several times everyminute and in doing so it is exposed to a cyclic compression which isexerted in the direction of the felt thickness. In addition, the felt isgradually permanently deformed until it is reduced to a state of reducedfunction. Felts of today's structure furthermore are liable to damage incase e.g. lumps of pulp pass through the press nip, as the compressiblefelt layer lacks the elastic properties necessary to take thedeformation stresses that are generated under these circumstances.Damage of this kind to the press felt or to the batt thereof oftenoccurs in the form of a razor-sharp slit in the crosswise direction ofthe felt, and the appearance of the slit suggests that the felt breakson account of a very high, localized tensile stress that is exerted inthe longitudinal felt direction. By increasing the thickness of the feltit is possible to increase the total compression without affecting theless compressible felt part. Although the result is that localizedcompression may be earier taken by the felt without damaging the cloth,it does, however, also bring about the disadvantage of reducing theopenness and rendering water through-flow more difficult. The increasedproportion of the batt included in the compressible part of the feltalso increases the plastic deformation of the felt, which means anincrease of the successive reduction of the permeability and at the sametime increased risks of damage.

SUMMARY OF THE INVENTION

The subject invention aims at eliminating this drawback by incorporatinginto the needled felt high-molecular elastomer materials which lendthemselves to stretching to at least twice their original length andthereafter, when the load on them ceases, rapidly resume substantiallytheir original length. The elastomer material should possess suchproperties that it imparts to the felt the necessary resilience duringthe entire serviceable life of the felt, also under the extremely severeconditions caused by high machine speeds and high squeezing pressures.

DETAILED DESCRIPTION OF THE INVENTION

Attempts have earlier been made to mix rubber materials into wovenconventional dryer felts and wet felts. However, these attempts havebeen limited to conditions that do not meet today's demands on machinespeeds and squeezing pressures. The earlier used rubber materials haveproved unsuitable for use in the manufacture of needled felts. Duringthe manufacturing operation of such needled felts the barbs of theneedles penetrate through the batt and the base weave to anchor the battfibres directly or indirectly to the base weave. This exposes thefibrous material to considerable mechanical stress. Ordinary rubbermaterials are immediately torn into pieces by the needle barbs. Inaddition, the conventional rubber materials have a comparatively lowinitial module which makes them unsuitable for use in connection withthe high machine speeds and operational tensions that are characteristicof modern papermaking machines. Furthermore, conventional rubbermaterials have limited resistance to the effects of ozone and oxidation.These materials age comparatively rapidly in the environment prevailingin papermaking machines.

Instead, in accordance with the teachings of the subject invention,high-molecular thermoplastic elastomers or urethane-based conventionalelastomers are used. Thermoplastic elastomers are materials that do notneed to be cross-linked. The materials soften upon temperature increasesand may easily be worked in the manner of conventional thermoplasticresins, e.g. be extruded into fibrous materials. Of the thermoplasticelastomers above all those of polyurethane and polyester types haveproved suitable for the purpose in accordance with the invention. Alsocross-linked urethane-based elastomers meet the requirements that areput on materials intended for use in felts that are applied on modernpapermaking machines. These materials possess excellent mechanicalproperties and good chemical stability. The elastomers may be stretchedto at least twice their original length and rapidly resume theiroriginal length, or near original length, when the load on them issubsequently relieved. A normal value of the elasticity of the materialis that after having been stretched to double its original length itreturns to 5% or less permanent elongation in approximately 5 secondsafter the load on it is removed. This does not, however, exclude thatelastic materials with properties deviating from this general value maybe considered suitable for felts in accordance with the invention.

More precisely, the subject invention thus is concerned with adewatering felt for use in the press section of papermaking machines,cellulose machines and similar machines, which felt is characterised inthat it comprises high-molecular thermoplastic elastomer materials whichare capable of being stretched to at least twice their original lengthand after relief of the load thereon, rapidly resume substantially theiroriginal length.

In accordance with another aspect of the subject invention, the feltcomprises high-molecular cross-linked urethane-based elastomer materialswhich are capable of being stretched to at least twice their length andthereafter, after relief of the load thereon, rapidly resumesubstantially their original length.

The dewatering felt in accordance with the invention has a conventionalbase. The latter may in the usual manner consist of a woven press feltor cloth but preferably of a needled felt with or without a base weave.The batt part of the needled felt may consist of one or several layersof varying fibre fineness. This conventional base structure includes, inaccordance with the teachings of the invention high-molecular elastomersin such quantities as to affect significantly the elastic properties ofthe felt.

Examples of various applications of the invention:

I. As the base weave of a needled felt woven endless in a four-shaftpattern so as to form a double-layer structure including two layers inthe weft direction, i.e. the machine direction. One of these layersconsists of yarns comprising a core of an elastomer polyurethanefilament about which is spun a polyamide thread. The second weft layerof the base weave consists of multifilaments or monofilaments and itswarp or crosswise yarns consist of monofilaments, none of them beinghighly elastic materials. The weight of the base weave is calculated to750 g/m² of which the high-elastic material is appr. 120 g/m². On top ofthe base weave is needled a batt having a surface weight of 450 g/m² andcomprising a mixture of polyamide fibres of 6 and 15 deniers.

II. The same structure of the base weave and the batt as in theaforegoing Example, except that the polyurethane filament about which isspun a polyamide thread is replaced by a filament, also in this case ofan elastic polyurethane but having no thread spun about it. In this casethe weight of the base weave is calculated to 750 g/m², of which theelastomer materials is appr. 200 g/m².

III. A base weave woven endless in a four-shaft, single-layer structurewherein every other thread in the warp as well as in the weft consistsof elastic polyurethane monofilaments alone or having other yarns spunabout them. Every other machine direction thread consists of a twistedmonofilament thread and every other crosswise thread of a singlemonofilament. The weight of the base weave is calculated to 400 g/m² ofwhich the elastomer material is appr. 200 g/m². On top of the base weaveis preferably needled a bat corresponding to the one mentioned inExample I.

IV. A base weave woven endless in a four-shaft, single-layer structurewherein the machine direction threads are twisted monofilaments and thecrosswise threads are single monofilaments. On top of this base weave isneedled a batt having a surface weight of 700 g/m² and comprising to 80%polyamide fibres of 6 and 15 deniers and to 20% highly elastic fibrousmaterials.

In the above Examples the elastomer material consists of threads or offibrous material. The material may also be included as one component ofthe bicomponent fibres.

Elastomer materials of polyurethane of conventional cross-linked type aswell as of thermoplastic type have proved to be very suitable for mixinginto dewatering felts in accordance with the subject invention. However,also other high-molecular thermoplastic elastomer materials may be used.As examples hereof may be mentioned styrene-based polymers,olefine-based polymers and ester-based polymers.

The elastic properties of the felt are affected by the quantity ofelastomer materials that is included into its basic structure. Alreadysmall quantities admixed thereto may give measurable physical changes.To achieve optimum effect it is, however, necessary that a certainproportion of elastomer material is included. It has been found that theadmixture in the base weave should amount to at least 15% of the weightof the base weave and to at least 10% of the total weight of the felt.

The use of thermoplastic elastomer materials and cross-linkedurethane-based elastomer materials in dewatering felts has severaladvantages over prior-art conventional products. Because of theincreased elastic properties of the felt the latter resumes its originalshape more easily after each passage through a press nip in addition towhich its successive, permanent deformation is decreased. The felttherefore retains its openness over a longer period of time. The abilityof the felt to resume its original shape after deformation alsodiminishes the risks of damage, e.g. in case a lump of pulp enters thepress nip. The elastic material of the felt in these cases take thedeformation stress and resumes its original shape, when the pulp lumphas passed the nip. Modern high-speed paper machines are very sensitiveto vibrations. The admixture of elastic materials into the dewateringfelt has a vibration-dampening effect on the entire press section of themachine. Because it is possible to keep the felt in accordance with theinvention more open, it is also easier to keep it clean. The increasedelasticity of the felt in addition brings about the advantage ofproviding wider press nips and consequently longer squeeze times.

The product in accordance with the invention as described above anddefined in the appended claims is referred to as a "dewatering felt".This concept is intended to include also other types of machine clothsused for dewatering purposes.

What is claimed is:
 1. A dewatering felt of needled textile fibrematerial for use in the press section of paper-making machines,cellulose machines and similar machines, characterised in that saidmaterial comprises high-molecular thermoplastic elastomer fibres whichmay be stretched to at least twice their original length and thereafter,after relief of the load thereon, rapidly return to essentially theiroriginal length.
 2. A dewatering felt as claimed in claim 1,characterised in that the thermoplastic elastomer fibres areurethane-based polymers.
 3. A dewatering felt as claimed in claim 1,characterised in that the thermoplastic elastomer fibres are ester-basedpolymers.
 4. A dewatering felt as claimed in claim 1, characterised inthat the thermoplastic elastomer fibres are styrene-based polymers.
 5. Adewatering felt as claimed in claim 1, characterised in that thethermoplastic fibres are olefine-based polymers.
 6. A dewatering felt asclaimed in any of the preceding claims characterised in that theelastomer fibres form threads.
 7. A dewatering felt as claimed in claim6, characterised in that the elastomer fibres are included as onecomponent of bicomponent fibres.
 8. A dewatering felt as claimed inclaim 1, comprising a batt and base structure characterised in that theelastomer material is included in the batt of the needled felt.
 9. Adewatering felt as claimed in claim 1, comprising a batt and a basestructure characterised in that the elastomer material is included inthe base structure of the needled felt.
 10. A dewatering felt as claimedin claim 1, comprising a batt and a base structure, characterised inthat the elastomer material of the base structure of the felt amounts toat least 15 percent by weight of the base structure and to at least 10percent by weight of the total weight of the felt.
 11. A dewatering feltof needled textile fibre material for use in the press section ofpaper-making machines, characterised in that said material compriseshigh-molecular cross-linked urethane-based elastomer fibres which arecapable of being stretched to at least twice their length andthereafter, after relief of the load thereon, rapidly return tosubstantially their original length.